1. Field of Use
The invention relates to the manufacture of cement or lime based slurries that can be used to stabilize and condition substrate soils and, more particularly, to the use of a novel apparatus for manufacturing slurries.
2. Related Art
In the construction industry, a stable rigid base is required for paving, building and parking structures, which requires the stabilization of the substrate soil. This stabilization may be accomplished by combining cement with the soil. Combinations of cement and soil are referred to as, but not limited to, soil cement, cement treated base, cement stabilized soil, and cement treated soil. The creation of soil cement involves the addition of specified amounts of cement per cubic unit of soil. The aforementioned soil is then graded and compacted to meet requirements specified in job plans and drawings. The cement treated soils are then allowed to cure, a chemical process whereby the cohesive material gains in strength and rigidity over time.
In other cases, lime may be used to treat the soil to form a stabilized base for paving, building and parking structures. Lime can be used in the form of quicklime (calcium oxide—CaO), hydrated lime (calcium hydroxide—Ca[OH]2) or lime slurry. Quicklime is manufactured by chemically converting limestone (calcium carbonate—CaCO3) into calcium oxide. Hydrated lime is created when quicklime chemically reacts with water. Lime slurry is a suspension of hydrated lime in water and can be formed from either hydrated lime or quicklime. Most lime material used for soil treatment is “high calcium” lime, which contains no more than 5 percent magnesium oxide or hydroxide. On some occasions, “dolomitic” lime is used. Dolomitic lime contains 35-46 percent magnesium oxide or hydroxide. Dolomitic lime can perform well in soil stabilization, although the magnesium fraction reacts more slowly than the calcium fraction. Lime is also sometimes used to describe byproducts of the lime manufacturing process (such as lime kiln dust), which do have as much reactive lime as the manufactured product but can still be used as a lime material to treat and stabilize a base for paving.
Lime materials can be used to treat the soil of the base in a number of ways. Quicklime, because it chemically combines with water, can be used to dry wet soils. When clays are present, hydrated lime will also chemically react with clay particles to provide further drying of the soil as well as chemically transforming them into a strong cementitious mix. After the initial mixing, the calcium ions (Ca++) from hydrated lime migrate to the surface of the clay particles and displace water and other ions. As a result, the soil becomes friable and granular, making it easier to work and compact. The Plasticity Index of the soil decreases dramatically as does its tendency to swell and shrink. When sufficient lime is added to raise the pH of the soil to above 10.5, clay particles will start to break down. Silica and alumina are released and react with calcium from the lime to form calcium-silicate-hydrates (CSH) and calcium-aluminate-hydrates (CAH). CSH and CAH are cementitious products similar to those formed in Portland cement. They form a matrix that contributes to the strength of lime-stabilized soil layers. As this matrix is formed, the soil is transformed from a sandy, granular material to a hard, relatively impermeable layer with significant load bearing capacity. The matrix formed is permanent, durable, and significantly impermeable, producing a structural layer that is both strong and flexible. As a result, in some cases lime may be used instead of cement to stabilize the substrate soil in preparation for paving or other construction.
Lime by itself can react with soils containing as little as 7 percent clay and Plasticity Indexes as low as 10. If the soil is not sufficiently reactive, lime can be combined with an additional source of silica and alumina. Such “pozzolans” include fly ash, and ground blast furnace slag. Fly ash is the most commonly used pozzolan. It is the finely divided residue that results from the combustion of pulverized coal in power plant boilers, which is transported from the combustion chamber by exhaust gases. Use of Lime Kiln Dust (LKD), which is the finely divided residue from the combustion of coal and the processing of limestone into lime in a lime kiln, is an increasingly popular alternative. LKD usually contains a significant amount of lime, alumina, and silica and thus is in essence a preblended mix of lime and pozzolan. Alkaline bypass dust (ABD) is a waste product from the cement manufacturing process in the clinker cooling phase of production that is very high in calcium oxide, but also contains some cement that will cure on its own. This provides ABD the benefits of lime treatment of the soil, while also creating soil cement.
One method commonly used to create cement or lime treated soil is a dry method. This generally involves using a pneumatic pumping apparatus to blow dry cement or lime onto a soil substrate and then mixing it into the soil. While commonly used, this process is considered undesirable due to the large amount of dust created by the blowing of fine cement or lime powder. This is especially undesirable in more populated areas. While using quicklime can minimize the dust creation problem to some extent, the fact that it requires 32 percent of its weight in water to convert to hydrated lime creates additional processing and handling concerns especially where drying is not required or there is no convenient water supply.
Another method to create soil cement or lime treated soil is to use a slurry. Cement/lime slurry is basically powdered cement or lime blended in water. There are various known methods for creating such slurries. The slurry preparation is placed over a substrate soil and then mixed in using mechanical means. Slurries are seen as a more desirable method of treatment than that of dry methods. However, slurry methods have proven to be very problematic in use. Cement slurry will harden in shipping vehicles if not removed in a timely manner. In addition, the cement or lime itself will start to separate or fall out of solution almost immediately after mixing with water. While lime slurries do not set, there is no fineness specification for lime products so they can have a very wide variety of particle sizes. This makes it more difficult to keep the lime suspended in the slurry. Lime slurries have been effectively limited to 35-45% of solids (e.g. hydrated lime, LKD, ABD) in water, even when using continuous mechanical mixing.
Methods to overcome the propensity of cement and lime particles to precipitate inside transportation vehicles include drum mixing and recirculation pumps. Both methods involve mechanical means to keep the solution moving so as to prevent the cement or lime from settling within the shipping vehicle. Should settlement occur, the resulting segregation of the water and cement or lime particles make it impossible to properly regulate the dispersion of the binary solution. This event also creates a great deal of maintenance problems for the vehicle operator. As a result of these limitations and despite the lack of dust creation, the use of slurries tends to be rare. The problematic properties of slurry have significantly limited its use and made other stabilization methods more desirable.
In addition to the settling problem, lime reacts with the water present in the slurry to rapidly heat the slurry. This occurs within about fifteen to twenty minutes after mixing and can result in the water in the slurry boiling off. Without sufficient water the lime material will clump and harden hampering its ability to be dispersed on the soil to be treated. The heating can even form steam pockets that in some cases have been known to blow the access hatch off a mixing truck that is being used to mix the slurry.
The conventional way to deal with this excess heat issue is to continue adding additional water to ensure a very high excess of water is always present. However, this results in lower and less consistent amounts of solids content in the final slurry. Since the final slurry has a less consistent amount of solids content from batch to batch, it is more difficult for the customer to reliably use the slurry to treat the base soil. The lower solids content leads to a viscosity for the slurry that is much closer to water, thus making it more difficult to keep the slurry from flowing out of position before it can be mechanically mixed into the soil. The excess water required to keep the lime in suspension also can raise the moisture level of the substrate soil above the desired range for creation of the treated base. Consequently, there is still a need for a composition and method to allow the use of lime slurries containing consistently high concentrations of solids.
As an alternative to using cement or lime to treat the soil base, a process called full depth reclamation can be used to provide a base for structures such as roads, parking lots, and other paved areas. This process involves grinding up and pulverizing the asphalt surface, cutting it to grade and blending it with the underlying base, subbase, and/or subgrade material. Cement and water is added to the combined materials to stabilize it much in the same way that cement can be added to substrate soil to created stabilized soil cement. Lime may also be used in this process to improve the properties of the soil/aggregate mixtures. The mixture is then compacted in place to form a stabilized substrate for the new paving. This process allows deteriorated roadways and other paved surfaces to be rebuilt without requiring lots of additional aggregate raw materials or having to cart away and dispose of the old asphalt and other materials. The process also allows roadways to be repaved and strengthened without changing the level of the grade, thereby avoiding issues where the pavement meets curbs and drain grates as well as avoiding changing the amount of clearance under overpasses on roadways. However, this process, because it involves the addition of cement and optionally lime to stabilize the base, runs into the same problems discussed above with respect to the application of cement and lime for soil stabilization.
While rare, thixotropic compounds have been occasionally used in connection with cement applications, although not in connection with soil cement slurries. Methylcellulose and related compounds have been used when pouring concrete underwater. In contrast to forming a gel and controlling hydration, the methylcellulose is added to increase internal cohesion and to thereby minimize the amount of concrete that is washed away by the surrounding water. This reduces the requirement that the structures be over engineered to account for the amount of sacrificial concrete that is washed away. Starch or other thickeners such as silica have been used in specific cement applications such as shotcrete, where dry or mixed concrete is sprayed onto walls and/or ceilings, and in thin set mortar. However, in these applications, the thickener is added to a mixture that is already very low in water content and viscosity, for purposes of increased self-adhesion, and does not lower the viscosity of a fluid, form a gel, reduce the tendency for disaggregation or settling out of suspended elements in a fluid, or increase the time before the mixture sets.